API interface definition in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an API interface in Android Kotlin development?

An API interface in Android Kotlin defines how your app communicates with a web service. It declares functions that represent HTTP requests, like GET or POST, to fetch or send data.

beginner

What Kotlin library is commonly used to define API interfaces for network calls?

Retrofit is a popular Kotlin library that helps define API interfaces. It uses annotations to describe HTTP methods and endpoints, making network calls easy and type-safe.

intermediate

Explain the role of annotations like @GET and @POST in API interface definition.

Annotations like @GET and @POST tell Retrofit what kind of HTTP request to make and which endpoint to call. For example, @GET("users") means a GET request to the "users" path.

intermediate

How do you define a function in an API interface to send a POST request with a JSON body?

You define a function with @POST annotation and use @Body parameter to pass the JSON data. For example: @POST("login") suspend fun login(@Body user: User): Response<Token>

intermediate

Why use suspend functions in API interface definitions with Retrofit and Kotlin?

Suspend functions allow network calls to run asynchronously without blocking the main thread. This keeps the app responsive while waiting for the server response.

Which annotation is used to define a GET request in a Retrofit API interface?

A @DELETE

B @POST

C @PUT

D @GET

The @GET annotation specifies that the function will perform an HTTP GET request.

In Retrofit, how do you pass data in the body of a POST request?

A Using @Body annotation

B Using @Query annotation

C Using @Path annotation

D Using @Header annotation

The @Body annotation is used to send data as the request body in POST requests.

What keyword is used in Kotlin to make a network call asynchronous in an API interface?

A async

B suspend

C await

D defer

The suspend keyword marks a function as asynchronous, allowing it to be called within coroutines.

Which library is commonly used to create API interfaces in Android Kotlin?

A Room

B Glide

C Retrofit

D Dagger

Retrofit is the standard library for defining API interfaces and making network calls.

What does the @Path annotation do in a Retrofit API interface?

A Replaces a part of the URL path

B Adds a query parameter

C Adds a header

D Specifies the request body

@Path replaces a placeholder in the URL path with a method parameter value.

Describe how to define a simple Retrofit API interface in Kotlin for a GET request to fetch a list of users.

Think about the function signature and the annotation needed.

Explain why suspend functions are important in API interface definitions when using Retrofit with Kotlin coroutines.

Consider what happens during network calls and UI updates.

Practice

1. What is the main purpose of defining an API interface in Android Kotlin using Retrofit?

easy

A. To store data locally on the device

B. To specify how the app communicates with web services

C. To create UI components for the app

D. To manage app navigation between screens

Solution

1. Step 1: Understand API interface role

   An API interface defines methods that describe how the app sends requests and receives responses from web services.

2. Step 2: Identify the correct purpose

   It is not related to UI, local storage, or navigation but specifically to communication with web services.

3. Final Answer:

   To specify how the app communicates with web services -> Option B

4. Quick Check:

   API interface = communication with web services [OK]

Hint: API interfaces define web communication methods [OK]

Common Mistakes:

• Confusing API interface with UI components
• Thinking API interfaces handle local data storage
• Assuming API interfaces manage app navigation

2. Which of the following is the correct way to define a GET request method in a Retrofit API interface?

easy

A. @GET("users") fun getUser(): Call<User>

B. @GET fun getUser(): User

C. fun getUser(): Call<User>

D. fun getUser() @GET("users"): Call<User>

Solution

1. Step 1: Recall Retrofit syntax for GET

   Retrofit uses @GET annotation with the endpoint path as a string parameter before the method declaration.

2. Step 2: Check method return type and annotation placement

   The method should return Call<User> and the annotation must be above the method signature.

3. Final Answer:

   @GET("users") fun getUser(): Call<User> -> Option A

4. Quick Check:

   @GET("endpoint") before method with Call return [OK]

Hint: Annotations go above method with endpoint string [OK]

Common Mistakes:

• Placing @GET after method name
• Omitting endpoint path in @GET
• Returning User directly instead of Call<User>

3. Given this Retrofit interface method:

@POST("login")
fun loginUser(@Body user: User): Call<Response>

What happens when loginUser is called?

medium

A. Throws a compile-time error due to missing annotation

B. Sends a GET request to 'login' without any data

C. Sends a POST request to 'login' with user data in the body

D. Returns user data without network call

Solution

1. Step 1: Analyze the method annotation and parameters

   The method uses @POST("login") which means it sends a POST request to the 'login' endpoint.

2. Step 2: Understand the @Body parameter

   The @Body annotation sends the 'user' object as the request body in the POST request.

3. Final Answer:

   Sends a POST request to 'login' with user data in the body -> Option C

4. Quick Check:

   @POST with @Body sends data in POST request [OK]

Hint: @POST with @Body sends data in request body [OK]

Common Mistakes:

• Confusing POST with GET request
• Ignoring the @Body annotation
• Assuming no network call happens

4. Identify the error in this Retrofit API interface method:

@GET("users/{id}")
fun getUser(@Path userId: Int): Call<User>

medium

A. The return type Call<User> is incorrect

B. The method is missing the @Body annotation

C. The method should use @POST instead of @GET

D. The @Path parameter name does not match the placeholder in the URL

Solution

1. Step 1: Check the URL placeholder and @Path parameter

   The URL has {id} but the parameter is named userId, so they do not match.

2. Step 2: Understand Retrofit @Path usage

   The name inside @Path must match the placeholder in the URL exactly to replace it correctly.

3. Final Answer:

   The @Path parameter name does not match the placeholder in the URL -> Option D

4. Quick Check:

   @Path name must match URL placeholder [OK]

Hint: Match @Path name exactly with URL placeholder [OK]

Common Mistakes:

• Using different names for @Path and URL placeholder
• Confusing @GET with @POST
• Adding unnecessary @Body in GET request

5. You want to define an API interface method that sends a POST request to "submit" with a JSON body and expects a Map response. Which is the correct method signature?

hard

A. @POST("submit") fun submitData(@Body data: Map): Call>

B. fun submitData(@Body data: Map): Map

C. @POST fun submitData(data: Map): Call>

D. @POST("submit") fun submitData(@Body data: Map): Map

Solution

1. Step 1: Confirm the annotation and endpoint

   The method must have @POST("submit") to specify the POST request and endpoint.

2. Step 2: Check parameter and return types

   The parameter should be annotated with @Body to send JSON data, and the return type should be Call> for Retrofit's asynchronous call.

3. Final Answer:

   @POST("submit") fun submitData(@Body data: Map): Call> -> Option A

4. Quick Check:

   @POST with @Body and Call return type [OK]

Hint: Use @POST with @Body and Call<> return type [OK]

Common Mistakes:

• Omitting endpoint path in @POST
• Returning Map directly instead of Call<Map>
• Missing @Body annotation on parameter